Optical disks such as compact disks (CDs), video compact disks (VCDs) and digital versatile disks (DVDs) are widely employed to store considerable digital data due to features of high capacity and portability. Accordingly, optical disk drives have become essential to personal computers. In order to enhance reproducing quality of the optical storage media, it is required to accurately and quickly read out the stored digital data. When an optical disk drive operates to read data from an optical disk, a laser beam emitted by a laser diode of the pickup head is focused onto the surface of the optical disk at a focusing point where the desired data is stored. There are plural pits and lands of various lengths on the surface of the optical disk. Due to the uneven profile, a portion of the laser beam will be reflected by the optical disk but the other will not. The laser beam portion reflected back to the pickup head and received by a photo sensor in the pickup head is converted into a corresponding radio frequency (RF) signal by subsequent circuit, which is provided for an actuating device of the optical disk drive.
Please refer to FIG. 1(a). A conventional digital data processing system for use in an optical disk drive comprises an analog-to-digital converter (ADC) 10, a retiming system circuit 11, a zero-crossing level tracking circuit 12 and a detector 13. The ADC 10 is used to periodically sample and convert the received RF analog signals into corresponding sampled signals in digital forms. The sampled signals are then checked by the retiming system circuit 11 to realize whether the features of the RF analog signals are retained. If not, the above procedures will be done again so as to obtain optimized sampled signals. Then, by means of the zero-crossing level tracking circuit 12, a zero crossing level is determined from the sampled signals. The zero crossing level is used as a reference level in the detector 13. According to the sampled signals' levels relative to the reference level, the detector 13 will output either a high level or a low level signal.
FIG. 1(b) illustrates another conventional digital data processing system. The digital data processing system comprises an analog-to-digital converter (ADC) 10, a phase-locked loop circuit 15, a zero-crossing level tracking circuit 12 and a detector 13. The ADC 10 is used to periodically sample and convert the received RF analog signals into corresponding sampled signals in digital forms according to the phase-lock loop circuit 15. The period for sampling the RF analog signals is determined by the phase-lock loop circuit 15 on the basis of waveform features and floating degrees of these RF analog signals. The operation principles of the zero-crossing level tracking circuit 12 and detector 13 are similar to those of FIG. 1(a).
The algorithm adopted by the zero-crossing level tracking circuit 12 to locate the zero crossing level will be illustrated as follows. Firstly, an initial reference level C is given. When a new sampled signal is generated, the total number of sampled signals generated within last certain time range and including the new sampled one are counted. The number of the sampled signals having levels greater than the initial reference level C is counted as A, whereas the number of the sampled signals having levels less than the initial reference level C is counted as B. If A>B, the reference level is adjusted to a value (C+D), where D is a constant positive value. On the contrary, if A<B, the reference level is adjusted to a value (C−D).
After the next sampled signal is received by the zero-crossing level tracking circuit 12, the above procedures are repeated to obtain a new reference level until the number of sampled signals having levels greater than the new reference level equals to the number of sampled signals having levels less than the new reference level. This new reference level is defined as the zero crossing level.
The above algorithm for locating the zero crossing level is considerably rough. For example, the zero crossing level may be converged to different values for different starting sampled signal. Referring to FIG. 1(c), when it is a sampled signal having a level higher than the initial reference level selected as the starting sampled signal, a relatively high first zero crossing level ZCL1 as shown will be obtained. On the contrary, when it is a sampled signal having a level lower than the initial reference level selected as the starting sampled signal, a relatively low third zero crossing level ZCL3 will be obtained. The realization of both of the first and the third zero crossing levels ZCL1 and ZCL3 complies with the above-described algorithm but deviates from the ideal second zero crossing level ZCL2 significantly. In addition, since the zero crossing levels ZCL1 and ZCL3 are close to either side of the sampled signals, the signals read by the optical pickup head might float due to noise resulting from unexpected interference during the data pickup process or trembling pickup head resulting from unstable current for actuating the pickup head. Accordingly, the first and the third zero crossing levels ZCL1 and ZCL3 are even unreliable so as to adversely affect the judgment of the detector 13.